The field of the present invention relates to optical devices incorporating distributed optical structures and temperature compensation. In particular, temperature-compensated planar waveguide optical apparatus, including spectral filters and temporal encoders, are disclosed herein.
An optical apparatus comprising a planar waveguide incorporating one or more distributed optical structures (i.e., one or more sets of diffractive elements) may be configured to provide a variety of optical functionality, including spectral filtering, temporal encoding, and others. Such devices, if single mode, may enable nearly complete control of amplitude and phase of optical signals to achieve filtering, encoding, routing, and other functions. Multimode devices may be employed for similar applications. Examples of such devices may be found in the prior applications cited hereinabove. However, the transfer functions of such devices exhibit shifts in wavelength with changes in the device's temperature, which may be caused, for example, by temperature-dependent changes in the refractive indices of the materials forming the planar waveguide and/or temperature-dependent changes in the mechanical dimensions of the waveguide (characterized by a so-called thermal expansion coefficient).
A typical value of the wavelength shift for a planar waveguide spectral filter implemented in a silica slab waveguide is 0.01 nm/° C. In many practical applications spectral filters have full-width-at-half-maximum (FWHM) bandpass windows of fractions of a nanometer and operate between about −5° C. and about 75° C. In such applications, the change in the ambient temperature within said limits will result in a wavelength shift of the center of the passband(s) of approximately 0.8 nm, which may be comparable to or larger than the passband of the filter. Such a relatively large temperature shift in passband center may necessitate external active thermal stabilization of the spectral filter, using thermal sensors, feedback controllers, and heating and/or cooling elements (such as Peltier elements). Use of such thermal stabilization apparatus adds substantial device cost, may lower device reliability, and may reduce wavelength accuracy. It may therefore be desirable to provide a planar waveguide optical apparatus with a designed temperature dependence for its spectral and/or temporal characteristics (which may include characteristics independent of temperature, or nearly so, or characteristics exhibiting some other desired temperature tuning characteristics). Devices with designed temperature dependence, particularly those with substantially reduced temperature dependence, may enable operation without thermal stabilization.